(1) Field of the Invention
The invention relates to a mechanism for an optical system, in particular to an optical system in a scanner.
(2) Description of the Related Art
In a typical scanner, two moving optical modules mounted with reflecting mirrors are used. The scanning light source is placed in one module. The scanning light ray is reflected by a mirror in the second module before incidence on a light sensor fixed on the frame of the scanner. Both modules are moving along the same track. It is desirable to maintain a fixed optical path between the light source and the sensor. This can be accomplished by moving both modules together, but with different speed. A speed ratio of 1:2 is often chosen. The difference in speed is obtained by choosing different diameters of the rollers driving the conveying belt for the two optical modules.
In a traditional scanner, the motor for driving the optical module is mounted on the frame of the scanner. When the driving motor is in operation, the motion may cause vibration of the frame of the scanner and create a great deal of noise. Such a noise is undesirable.
It is also desirable to miniaturize the size of a scanner.
An object of this invention is to reduce the noise of the dual speed modules. Another of this invention is to reduce the thickness and hence the size of a scanner with dual speed modules.
These objects are achieved by mounting the driving motor on one of the modules on the same moving carriage. Since the motor is not directly mounted on the frame, the frame does not vibrate as violently and the noise is reduced. In addition, by not mounting the motor on the frame, no space need be allowed for the motor in the vertical direction. Thus the frame of the scanner can be thinner and hence smaller.
Two moving optical modules are used. One of the moving optical modules, the first module, has two driving wheels. One of the wheels rubs against a first belt stretched between two opposite walls of the scanner frame. When this wheel turns in a clockwise direction, the friction causes this first module to move toward the right hand direction.
The second optical module is pulled by a second belt by the second driving wheel toward the first optical module by means of a second belt which loops from one side of the second optical module through the second driving wheel to the other side of the second optical module. When the second driving wheel rotates in the same clockwise direction as the first driving wheel, the second optical module tends to move toward the left side but is overcome by right side pull of the first optical module. The net pull results in a reduced speed right hand movement of the second optical module.
On the other hand, when the second driving wheels rotates in an opposite direction (i.e. counter-clockwise direction) to the first driving wheel, the second optical module moves twice as fast as the first optical module. Thus, the two optical modules move in different speed.